This invention relates to a method of depositing a refractory metal on a semiconductor substrate and, more particularly, to such a method wherein a passivation material is also deposited around the periphery of the metal.
In the semiconductor industry it is often necessary to deposit a refractory metal on a predetermined surface area of a semiconductor substrate and to also deposit a passivation material around the periphery of that metal. As an example, Schottky diodes include a metal contact on the surface of a silicon substrate to form a rectifying junction. The passivation material surrounds the edge of this contact to enhance the forward and reverse operating characteristics of the diode. When manufacturing these diodes, it is usual to coat the surface of the substrate with a passivation material, form a window in the passivation material to expose a predetermined surface area of the substrate and deposit the metal on this surface area.
Various passivation materials have been utilized including silicon dioxide (SiO.sub.2), semi-insulating materials such as oxygen doped polycrystalline silicon (SIPOS) and combinations thereof. The use of oxygen doped polycrystalline silicon either alone or covered by a layer of silicon dioxide has come to be preferred because of its enhanced passivation properties. For providing the contact, the use of refractory metals has been preferred because of their thermal stability and, recently, the use of tungsten (W) and molybdenum (Mo) deposited by Chemical Vapor Deposition (CVD) techniques have become most preferred.
The use of a CVD technique provides significant advantages over the more usual evaporation and sputtering deposition techniques. First, the metal adheres better to the silicon substrate due to the chemical interaction between the metal and the silicon. Second, a desirable metal silicide is automatically formed beneath the substrate surface adjacent the metal. This eliminates the need for high temperature alloying of the deposited metal and also avoids the exposure of the metal to the corrosive environment used in such alloying. Third, the refractory metal is deposited on the silicon, but not the usual outer oxide of the passivant system. This enables the refractory to be completely covered by a corrosion resistant film.
When forming schottky diodes using oxide or a combination of oxygen doped polycrystalline silicon and oxide in combination with a refractory metal contact deposited by CVD techniques, several problems have been realized. A most critical problem involves the formation of a severe undercut in the silicon substrate that is formed at the edge of the window where the silicon meets the oxide layer or where the oxygen doped polycrystalline silicon meets the oxide layer, depending upon the particular passivation material. This undercut is formed when the metal is deposited on the substrate and it severely degrades the forward and reverse operating characteristics of the device.